Bearing materials used for rolling bearings, etc., are required to have long rolling contact fatigue lives. The B10 life is used as one of evaluation methods of the rolling contact fatigue life. The B10 life refers to the total number of loadings until cumulative failure probability is statistically assumed to reach 10%. In general, it is well known that the rolling contact fatigue life of the bearing is affected by hard oxide-based nonmetallic inclusions existing in the material. Therefore, hitherto, improvement of the rolling contact fatigue life has been intended by reducing the amount of oxygen in the material and, therefore, by achieving reduction of the amount of the oxide-based nonmetallic inclusions. At present, it has become possible to reduce the amount of oxygen in the material to 10 ppm or less on a weight ratio basis with the help of the advance of refining technology. However, the method for improving the rolling contact fatigue life by reduction of oxygen has nearly reached its limit. Furthermore, when the amount of oxygen is reduced to 10 ppm or less, the manufacturing cost is increased and, therefore, it is difficult to apply to steels for general purpose use.
Recently, suggestions intended for further improvement of the rolling contact fatigue life have been made. For example, in Japanese Unexamined Patent Application Publication No. 3-126839, a bearing material for realizing a long life by controlling the number of oxide-based nonmetallic inclusions in a unit area or unit volume has been disclosed. In Japanese Unexamined Patent Application Publication No. 5-25587, a bearing material for realizing a long life by controlling a predicted maximum diameter of the oxide-based nonmetallic inclusion estimated with statistics of extremes has been disclosed. However, regarding super clean steel in which the amount of oxygen has been reduced to nearly the limit value of 10 ppm, the relationship between the size and the number of the oxide-based nonmetallic inclusions is not completely clear.
Japanese Unexamined Patent Application Publication No. 9-291340 has focused attention on the thickness and the number of the sulfide-based nonmetallic inclusions in steel, and on the predicted maximum diameter of the oxide-based nonmetallic inclusion. The steel for bearings which has a long life by controlling the number of the sulfide-based nonmetallic inclusions having a thickness of 1 μm or more at 1,200 or less in an inspection area: 320 mm2, and/or the predicted maximum diameter of the oxide-based nonmetallic inclusion at 10 μm or less in the inspection area: 320 mm2 has been disclosed. However, regarding the technique disclosed in Japanese Unexamined Patent Application Publication No. 9-291340, the B10 life of 5×107 or more has not been achieved with respect to the one in which the maximum diameter of the oxide-based nonmetallic inclusion is 10 μm or more. The aforementioned conventional technique is based on reduction of the number or the maximum diameter of the oxide to an extremely low value. In order to realize such super clean steel, a specific secondary refining process, such as ladle refining, is required. There are restrictions on the manufacturing process and the iron and steel maker which can carry out those in practice. Since the refining cost of steel is increased by a large degree, it is difficult to enlarge application to practical steels for general purpose use.
It is an object of the present invention to provide a long-life bearing material for general purpose use with no restriction on the refining process and no increase in the manufacturing cost.